This project will use anuran amphibians as a model system to investigate the interaction of vertebrate communication behavior and hormonal control systems. Behavioral manipulations will test the hypothesis that repeated exposure to conspecific communication signals increases plasma sex steroid levels and changes the anatomical characteristics of three neurochemical systems in brain thought to be involved in hormonal regulation and reproductive control: the gonadotropin releasing hormone (GnRH) centers in the medical septal area and preoptic area expressing the mammalian LHRH form of this peptide, the tegmental GnRH center expressing the chicken II form of GnRH, and the tyrosine hydroxylase (TH) populations in the basal forebrain. Behavioral and hormonal manipulations will then test between two competing hypotheses: that any CNS changes observed are due to feedback from sex steroids released by exposure to conspecific calls vs. that CNS changes are directly due to sensory stimulation of these areas. In addition, neuroanatomical tracing experiments and immunohistochemistry will determine the connections of the medial septal LHRH areas and the tegmental chicken II GnRH area to determine which GnRH populations send efferents to the median eminence and neurohypophysis and hence may control sex steroid secretion from the gonads. Finally, sensory inputs to the medial septal area LHRH and tegmental chicken II GnRH centers will be examined to test the hypothesis that they, like the preoptic area and ventral hypothalamus centers previously examined, receive auditory input (conveying formation about communication signals) in a convergent pattern from the thalamus and isthmal tegmentum. The anatomical results will be related to the behavioral results to test the hypotheses: that both LHRH and chicken II GnRH neurons contribute to neurosecretory functions, and hence, the control of sex steroid secretion; that both forms are equally influenced by the reception of conspecific calls; and that both neurosecretory and non-neurosecretory subpopulations of each form, if such subdivisions exist, are equally influenced by the reception of conspecific calls. The project will enhance the understanding of sensory-endocrine interactions that are fundamental to a variety of natural behaviors with clinical significance, including reproduction and stress.